The present invention relates to optical display apparatus particularly, although not exclusively, for use as a Head-Up-Display in a fighter aircraft.
Head-Up-Display (HUD) optics have been known for many years. The conventional form of a HUD optic is a large lens which magnifies and collimates the display on a small cathode ray tube. This collimated display is then reflected off a partially-reflective combiner situated immediately in front of the pilot. The pilot thus sees information (navigation, engine, weapon etc) superimposed on his line of sight and focussed at infinity. Thus, the pilot can, therefore, assimilate essential information without averting his gaze from the outside world or refocussing his eyes. The latter advantages eliminate the approximately two seconds of `dead` time that are usually required for averting gaze, focussing on instruments, assimilating information, re-directing gaze to the outside world and refocussing at infinity. This `dead` time could be extremely dangerous in a fast, low-flying fighter aircraft.
Conveniently, conventional HUD optics comprise folded collimating lenses of the Petzval form, that is two basic groups of positively powered optics separated by a long air-gap in which the folding means (usually a plane mirror) is situated. The folding of the optics enables the mechanical axis of the c.r.t. to be approximately parallel to the over-the-nose sight line of the pilot. The depth or profile depth of a conventional HUD optic is nominally only a little less than the diameter of the HUD exit lens and has not been a problem to date in terms of fit into the aircraft. Modern display technology has, however, enabled large, rugged, flat panel displays to be fitted to aircraft and these require a large area of cockpit facia space. Thus it is now required that HUD optics be substantially reduced in depth in order to provide adequate visibility of these flat panel displays. The requirement for a reasonable instantaneous field of view (IFOV) from the HUD, defined only by simple trigonometry involving the size of the HUD exit lens and its distance from the pilot's eyes, has not been relaxed. There are, therefore, apparently contradictory requirements that the HUD exit lens should be large in diameter (in excess of say 160 mm) but the profile depth of the optic, e.g. the actual physical depth of the box in which the components (c.r.t., folding mirror and collimating lenses) of the optical module are to be accommodated, should be very much less than the exit lens diameter, say 60% of this diameter. This is clearly difficult if not impossible with a conventional optical design.
An aim of this invention is to overcome the beforementioned problems by the provision of a head-up display optic which has a profile depth which is substantially and preferably much less than the diameter of the exit lens.